Matrix: Add python dep. for URL preview during installation

seqs/matrix.sh

@@ -98,7 +98,7 @@ step_4() {
   exe source "${MATRIX_HOME}/env/bin/activate"
   exe pip install --upgrade pip
   exe pip install --upgrade setuptools
-  exe pip install matrix-synapse[postgres]
+  exe pip install matrix-synapse[postgres] lxml
 }
 
 step_5_info() { echo "Create default configuration and folder structure"; }